The existance of liquid crystalline materials has been recognized since the late 1800's. The terms "liquid crystal" or "mesogen" refer to a number of states of matter which lie between solid crystals and isotropic liquids, the latter being randomly ordered. Liquid crystalline materials possess some structural characteristics of crystals, yet they may be viscous or quite mobile liquids.
The varying degrees of order which are possessed by liquid crystals give rise to three distinct types of structures called mesophases. A liquid crystal, when in the crystalline state, has a three-dimensional uniform structure with orientational and positional order. As the crystal is heated, it may initially lose one dimension of its positional order. This is referred to as the smectic mesophase, a phase in which the liquid crystal retains the orientational order of the crystalline state, as well as two-directional positional order.
With further heating, the liquid crystal can convert to the nematic mesophase. In this phase, the remaining positional order is lost and the liquid crystalline material retains only the one-directional orientational order of the crystalline state. The molecular order of nematic mesophases is characterized by orientation of the molecules along an axis which coincides with the long axis of the molecules. The centers of gravity of the molecules are arranged randomly so that no positional long-range order exists.
In the cholesteric mesophase, the molecular order is characterized by orientation of the molecules along an axis which coincides with the long molecular axis as in a nematic phase; however, the axis changes direction in a continuous manner along a second axis perpendicular to the first. This results in a long-range helicoidal ordering of molecules that gives rise to the unusual optical properties characteristic of cholesteric mesophases.
The term "cholesteric" is primarily of historical significance because the first discovered liquid crystalline material that exhibited a cholesteric mesophase was cholesteryl benzoate. It has long been recognized, however, that the presence of the cholesterol moiety is not required for formation of a cholesteric mesophase. All that is required is a nematic component and a chiral or optically active component. These components can be incorporated in one molecule, as in cholesteryl esters, or they can be separate molecules. Thus, a nematic mesophase can be "doped" with a small amount of optically active material to generate a cholesteric mesophase, more accurately termed a "twisted nematic" mesophase.
The optical properties characteristic of cholesteric mesophases include the selective reflection of light in the infrared, visible or ultraviolet regions, circular dichroism and a high degree of optical rotary power, all of which are well known in the art. Accordingly, the optical response of a cholesteric mesophase will include all the optical properties of a cholesteric mesophase.
One optical property, the ability of cholesteric mesophases to selectively reflect light in the infrared, visible or ultraviolet region, is useful in characterizing the structure of cholesteric mesophases. The wavelength of maximum reflection .lambda..sub.R is directly dependent on the helical pitch (P, the distance the helix takes to repeat itself) and the average index of refraction (n) of the cholesteric mesophase by .lambda..sub.R =nP. The pitch can be quite sensitive to changes in temperature or composition of the mesophases and, as a result, these functions have been used to control the wavelength of maximum reflection. Mesophases of cholesteryl alkanoates typically show a decrease in .lambda..sub.R with increasing temperature; thus a cholesteric mesophase reflecting red light will shift the reflection band toward the violet with increasing temperatures.
The ability of a cholesteric mesophase to reflect light also is dependent upon the alignment or texture of the cholesteric mesophase. As is well known in the art, there are three common textures for cholesteric mesophases. The most easily characterized texture is the cholesteric planar texture where the helicoidal ordering is aligned such that the pitch axis is perpendicular to the plane of the film surface. This planar texture can reflect visible light under certain circumstances resulting in the formation of brilliant colors. The homeotropic or fingerprint texture has the helicoidal pitch axis aligned parallel to the plane of the film surface. Mesophases with a large pitch exhibit a finterprint-type pattern visible under an optical microscope. In the focalconic texture there is no preferential alignment of the helicoidal axes in any one direction in the microdomains of the cholesteric mesophase; therefore, only a random scattering of light is observed and no colors are formed from reflection. This texture is most readily achieved by cooling a cholesteric material from the isotropic state into the cholesteric state without disturbing (mechanically shearing) the mesophase. These textures are useful in controlling the optical responses of cholesteric mesophases, as will be discussed in more detail herein.